Ferromagnetic resonance measurement

ABSTRACT

A method of measuring ferromagnetic resonance properties of a structure including a non-ferrous material reinforced with steel members, the method includes applying a magnetic field to the structure using a magnet located adjacent the structure, directing microwave radiation towards the structure, and detecting the propagation of microwave radiation that is transmitted or reflected by the steel members, the measuring being made with structure in situ.

BACKGROUND OF THE INVENTION

The present invention relates to a ferromagnetic resonance measurement.

A few elements, notably iron, cobalt and nickel show the effect ofspontaneous magnetisation, which is termed ferromagnetism. In aferromagnetic material atoms effectively act as atomic bar magnets whichinteract co-operatively so that large groups of atoms within a structurehave a common orientation of their magnetism. In a quantum mechanicaldescription, the alignment of magnet moments is ascribed to the exchangeinteraction, which energetically favours magnetic order.

Ferromagnetic resonance measurements are conceptually similar to nuclearmagnetic resonance measurements, which form the basis of magneticresonance imaging (MRI) scanners. A ferromagnetic sample is located in astrong magnetic field. The effect of the strong magnetic field is toalign the atomic magnetic moments in a single orientation and to alterthe energy levels of excited states of the atoms. Microwave radiation ata predetermined frequency is directed at the sample. The strength of themagnetic field is increased gradually, thereby altering the degree ofalignment of the atoms and modifying the energy levels of the excitedstates of the atoms. When an energy level of an excited state is equalto the energy of the incident microwave photons, the microwave radiationwill be resonantly absorbed by the ferromagnetic material.

The amount of microwave radiation absorbed by the sample is monitoredusing a microwave detector. The values of magnetic field strength whichgive rise to absorption of the microwave radiation are indicative of thestructure of the sample being tested.

In known ferromagnetic resonance measurements, a sample of a material tobe tested is located within a resonant cavity, the resonance of thecavity being selected for the frequency of microwave radiation that isto be directed at the sample. The resonant cavity enhances the signal tonoise ratio of the ferromagnetic resonance measurement.

Recently, a modified ferromagnetic resonance measurement has beendeveloped wherein a single value, magnetic field is applied to a sample,and microwave radiation directed at the sample is swept over a range offrequencies (M. E. Unwin et al, A novel broadband ferromagneticresonance spectrometer, Journal of Magnetism and Magnetic Materials,205(1999) 199–208). The sample is located in a waveguide in such a wayas to isolate the microwave radiation so that it travels through thesample in one direction only. This arrangement is based upon a resonantisolator, a microwave circuit device which allows microwave propagationin one direction only.

A disadvantage of known ferromagnetic resonance measurement methods isthat they require a sample to be located in a resonant cavity or in awaveguide. Thus, a ferromagnetic resonance measurement of a large samplecannot be carried out, unless part of that sample is removed and locatedwithin a resonant cavity or waveguide.

It is an object of the present invention to provide a ferromagneticresonance measurement method which overcomes or mitigates the abovedisadvantage.

BRIEF SUMMARY OF THE INVENTION

According to the invention there is provided a method of measuringferromagnetic resonance properties of a structure comprising anon-ferrous material reinforced with steel members, the methodcomprising applying a magnetic field to the structure using a magnetlocated adjacent the structure, directing microwave radiation towardsthe structure, and detecting the proportion of microwave radiation thatis transmitted or reflected by the steel members, the measurement beingmade with the structure in situ.

The inventors have realised that the ferromagnetic resonance signal fromnon-corroded steel is very different from that of rust, and thatconsequently the integrity of a structure comprising non-ferrousmaterial reinforced with steel members can be detected using aferromagnetic resonance measurement.

The invention provides a quick and direct non-destructive method ofdetermining the degree of corrosion of steel members located inside anon-ferrous material.

The term non-ferrous material is not intended to exclude a materialwhich contains a small amount of ferrous substances. Instead, the termnon-ferrous material is intended to mean that the amount of ferroussubstances within the material is sufficiently low that it does notaffect significantly the measurement according to the invention.

Suitably, the magnetic field is held substantially constant, microwaveradiation over a range of frequencies is directed at the structure, andone or more properties of the spectrum of microwave absorption withinthe steel members is determined as a function of frequency.

Suitably, the shape of the microwave absorption spectrum is determinedAlternatively, the frequencies at which maxima of microwave absorptionoccur may be determined.

Suitably, the microwave radiation is swept over the range offrequencies.

Suitably, the microwave radiation is provided as, a pulse whichincorporates the range of frequencies.

Preferably, an antenna is used to direct the pulse of microwaves at thestructure, and the same antenna is subsequently used to detect thatproportion of the pulse of microwaves that is transmitted or reflectedby the steel members.

Suitably, a spectrum analyser is used to determine the spectrum ofmicrowave absorption within the steel members as a function offrequency.

Alternatively, a spectrum analyser is used to determine the frequenciesat which maxima of microwave absorption occur.

Suitably, the frequency of the microwave radiation is held substantiallyconstant and the strength of the magnetic field applied to the structureis varied using an electromagnet, the shape of the spectrum of microwaveabsorption within the steel members being determined as a function offield strength.

Suitably,the frequency of the microwave radiation is held substantiallyconstant and the strength of the magnetic field applied to the structureis varied using an electromagnet, the magnetic field strength at whichmaxima of microwave absorption occur within the steel members beingdetermined.

Preferably, the method is calibrated by carrying out a series offerromagnetic resonance measurements of structures having known amountsof corrosion, an unknown amount of corrosion of a structure beingdetermined by carrying out a ferromagnetic resonance measurement of thatstructure and comparing the results of the measurement with thecalibration measurements.

Preferably, the microwave radiation is fixed at, or has a centralfrequency, at or around 1 GHz.

Preferably, the magnetic field is sufficiently strong to saturate themagnetic state of the steel members.

Preferably, the magnetic field strength is fixed at, or has a centralfrequency, at or around 0.1 Tesla.

The structure preferably comprises reinforced concrete.

Preferably, a background measurement is taken with no magnetic fieldapplied to the structure, and the results of the background measurementare compared with a subsequent ferromagnetic resonance measurement ofthe structure, in order to enhance the signal to noise ratio of themeasurement.

A specific embodiment of the invention will now be described by way ofexample only, with reference to the accompanying FIGURE which is aschematic diagram representing a ferromagnetic resonance measurementaccording to the invention.

DETAIL DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a reinforced concrete all comprises steel bars 1located within concrete 2. The concrete 2 is a non-ferrous material.

A permanent magnet 3 comprising a north pole 4 and south pole 5 islocated against the reinforced concrete wall. A magnetic field producedby the magnet 3 passes through the concrete 2, into the steel bars 1,along the steel bars 1 and back though the concrete 2. A single magneticfield line 6 indicative of the magnetic field generated by the magnet 3is shown in FIG. 1.

A microwave source 7 generates a white beam of microwaves (i.e.microwaves over a range of wavelengths) which is directed using anantenna 8 into the concrete 2. The microwaves arc reflected by the steelbar 1, pass back through the concrete 2 and are detected by a secondantenna 9. The detected microwaves are analysed using a spectrumanalyser 10.

The spectrum analyser 10 is used to determine the frequency spectrum ofthe microwave absorption. The shape of this spectrum is characteristicof steel and/or rust present within the reinforced concrete wall, andthe amplitude of the absorption is directly related to the amounts ofsteel and rust present.

The spectrum analyser 10 is calibrated by testing reinforced concretehaving different known levels of corrosion, and storing resultingmicrowave spectra. Following calibration, the amount of corrosion in areinforced concrete wall is determined by comparing the measuredabsorption spectrum with the calibration spectra.

The microwave source 7 may generate tho White beam of microwaves as apulse. Where this is done only a single antenna 8 is required, theantenna 8 being connected to the microwave source 7 during pulsegeneration, and connected to the spectrum analyser 10 during pulsedetection. A further advantage of the microwave pulse is that itautomatically provides broad band of microwave.

The microwave source 7 may provide a continuous beam of microwaves sweptover a range of frequencies. Where this is done a spectrum analyser isnot required, since the microwave frequency directed at the reinforcedconcrete wall at any given time is known.

In a further alternative arrangement, microwaves having a fixedfrequency may be used and the strength of the magnetic field applied tothe reinforced concrete varied. In order to vary the magnetic field themagnet 3 must be an electromagnet rather than a permanent magnet.

The magnetic field preferably saturates the magnetic state of the steel.A preferred value of the magnetic field strength is around 0.1 Tesla atresonance.

A preferred central frequency for the microwaves is around 1 GHz.

The absorption spectrum of microwaves that are transmitted by thereinforced concrete, rather than reflected, may be measured. Ameasurement of this type is mathematically equivalent to a reflectionmeasurement.

In order to reduce the effect of background noise, a microwave spectrummay be determined without the magnet 3 in place. This backgroundmeasurement may be compared with a subsequent measurement made with themagnet 3 in place.

The microwave source 7 may be arranged to direct microwaves at theconcrete 2 without requiring an antenna.

1. A method for non-destructive determination of the degree of corrosionof steel members of a structure comprising a non-ferrous materialreinforced with said steel members, the method comprising measuringferromagnetic resonance properties of the structure by applying asubstantially constant magnetic field to the structure using a magnetlocated adjacent the structure, directing microwave radiation over arange of frequencies towards the structure, and detecting the proportionof microwave radiation that is transmitted or reflected by the steelmembers, wherein the measurement is made with the structure in situ andwherein one or more properties of the spectrum of microwave absorptionwithin the steel members are determined as a function of frequency.
 2. Amethod according to claim 1, wherein the shape of the microwaveabsorption spectrum is determined using a spectrum analyzer.
 3. A methodaccording to claim 1, wherein the frequencies at which maxima ofmicrowave absorption occur are determined.
 4. A method according toclaim 1, wherein the microwave radiation is swept over the range offrequencies.
 5. A method according to claim 1, wherein the microwaveradiation is provided as a pulse which incorporates the range offrequencies.
 6. A method according to claim 5, wherein an antenna isused to direct the pulse of microwaves at the structure, and the sameantenna is subsequently used to detect that proportion of the pulse ofmicrowaves that is transmitted or reflected by the steel members.
 7. Amethod according to claim 5, wherein a spectrum analyzer is used todetermine the spectrum of microwave absorption within the steel membersas a function of frequency.
 8. A method according to claim 5, wherein aspectrum analyzer is used to determine the frequencies at which maximaof microwave absorption occur.
 9. A method according to claim 1, whereinthe frequency of the microwave radiation is held substantially constantand the strength of the magnetic field applied to the structure isvaried using an electromagnet, the shape of the spectrum of microwaveabsorption within the steel members being determined as a function offield strength.
 10. A method according to claim 1, wherein the frequencyof the microwave radiation is held substantially constant and thestrength of the magnetic field applied to the structure is varied usingan electromagnet, the magnetic field strength at which maxima ofmicrowave absorption occur within the steel members being determined.11. A method according to claim 1, wherein the method is calibrated bycarrying out a series of ferromagnetic resonance measurements ofstructures having known amounts of corrosion, an unknown amount ofcorrosion of a structure being determined by carrying out aferromagnetic resonance measurement of that structure and comparing theresults of the measurement with the calibration measurements.
 12. Amethod according to claim 1, wherein the microwave radiation is fixedat, or has a central frequency, at or around 1 GHz.
 13. A methodaccording to claim 1, wherein the magnetic field is sufficiently strongto saturate the magnetic state of the steel members.
 14. A methodaccording to claim 1, wherein the magnetic field strength is fixed at,or has a central frequency, at or around 0.1 Tesla.
 15. A methodaccording to claim 1, wherein the structure comprises reinforcedconcrete.
 16. A method according to claim 1, wherein a backgroundmeasurement is taken with no magnetic field applied to the structure,and the results of the background measurement are compared with asubsequent ferromagnetic resonance measurement of the structure, inorder to enhance the signal to noise ratio of the measurement.
 17. Amethod for non-destructive evaluation of a structure comprising anon-ferrous material reinforced with steel members, the methodcomprising determining one or more properties of a spectrum of microwaveabsorption within the steel members as a function of frequency byapplying a substantially constant magnetic field to the structure usinga magnet located adjacent the structure, directing a pulse of microwaveradiation towards the structure using an antenna, and detecting theproportion of the pulse of microwave radiation that is transmitted orreflected by the steel members with said antenna to determine the degreeof corrosion on said steel members, the measurement being made with thestructure in situ.
 18. A method according to claim 17, wherein aspectrum analyzer is used to determine the spectrum of microwaveabsorption within the steel members as a function of frequency.
 19. Amethod according to claim 18, wherein a spectrum analyzer is used todetermine the frequencies at which maxima of microwave absorption occur.